In general, this novel, and economically important, result is obtained by milling or otherwise comminuting raw coal until it has been reduced in particle size to ca. 250 .mu.m.times.0 (.mu.m equals micrometer or micron). The raw coal is then slurried in an aqueous liquid, typically clean water; and comminution of the raw coal is continued until the raw coal has been resolved into separate, particulate phases of coal and mineral matter. After this comminution step is completed, a large amount of an agglomerating agent is added to the slurry with agitation; agitation of the slurry is continued until the coal particles have dissociated from the mineral matter and aqueous phases of the slurry and coalesced into agglomerates of product coal; and the agglomerates are recovered from the slurry (there is virtually 100 percent recovery of the carbonaceous material in this separation).
A product coal with an even lower ash content than is available from following the steps identified above can be produced by redispersing the product coal agglomerates in clean water and repeating the agglomeration and collection steps. This sequence can be repeated as many times as wanted although it is presently believed that the benefits obtained by proceeding beyond the third collection step will in general not justify the expense of doing so.
No additional milling is required in the second product coal recovery stage (dispersion, agglomeration, and recovery steps) just discussed or in subsequent repetitions of this sequence of steps. Consequently, the elimination of additional ash afforded by the second (and any subsequent) stages can be effected inexpensively and with only modest expenditures of energy.
Still another technique that can be employed to reduce the ash content of the product coal obtained in the initial (or a subsequent) agglomeration and separation of the product is an acid leach of the product coal.
All of the above-discussed process steps can be carried out at ambient pressure and at ambient temperatures (preferably 70.+-.10.degree. F. (21.2.+-.5.6.degree. C.)).
The process described above can be used to prepare fuels which can compete directly with Bunker C and residual crude oils and synthetic coal fuels which have been successfully employed to fuel gas turbine engines. The flame characteristics of these novel fuels lie between those of flames obtained by burning natural gas and No. 2 fuel oil, respectively.
Specifically, product coals with ash contents of substantially less than 1.0 weight percent have been produced by the foregoing process with demonstrated repeatability from a number of quite different coals. These fuels typically have the following characteristics:
______________________________________ Particle Size down to 4 .mu.m .times. 0 Ash down to 0.22 wt % Moisture below 5 wt % BTU/lb in the range of 15,000 BTU Percent Yield approaching 100% ______________________________________
Small particle size is an important contributing factor to the usefulness of a coal-type fuel. The process described above is eminently capable of generating such fuels as is shown by the foregoing tabulation.
As indicated above, the raw coal being processed into a low ash fuel as disclosed herein is preferably first milled or comminuted while in a "dry" state, formed into an aqueous slurry, and then subjected to further size reduction. Unexpectedly, it has been found that this is economically advantageous while the efficiency of the process is not adversely effected by the dry milling contrary to what is stated in U.S. Pat. No. 4,186,887 which was issued Feb. 5, 1980, to Douglas V. Keller, Jr., et al and which discloses an agglomeration type coal recovery process which, in certain respects, is like the fuel preparation process described herein.
The raw coal is reduced to a top size of ca. 85 percent 250 microns.times.0 by dry milling, as indicated above, and subsequently ground to an ultimate top size 30 .mu.m with a particle size of 85 percent 15 .mu.m.times.0 being preferred. In some cases the size distribution of the comminuted raw coal limits the maximum degree of ash reduction. The finer the particles the more mineral matter that can be separated.
Another technique that I can advantageously employ to increase the efficacy of the novel fuel preparation process described above involves the addition of milling aids in small amounts to the raw coal in the second of the comminution steps. Such additives perform one, or both, of two important functions--promotion of particle dispersion, which results in more efficient milling, and protection of freshly exposed particle surfaces against oxidation. This facilitates the subsequent interaction between the coal particles and the agglomerant and thereby promotes more efficient separation of the coal from the mineral matter and liquid phases of the slurry when the separation and agglomeration of the coal particles is carried out.
The particular additives that are employed depend upon the particular coal being cleaned. Additives that have been employed to advantage include: 1,1,2-trichloro-1,2,2-trifluoroethane; OT-100, a dioctyl ester of sulfosuccinic acid marketed by American Cyanamid as an ionic surfactant; Surfynol 104E, a tertiary acetylenic glycol marketed by Air Products and Chemicals, Inc. as a nonionic surfactant; and Triton X-114, an octyl phenol with 7-8 oxide groups marketed by Rhom & Haas Co. as a nonionic surfactant.
Coal particle surface protection is obtained by adsorbing monolayers of the milling additive onto the surfaces of the coal particles in the second (wet) of the milling steps. This requirement can be met by introducing the milling additive into the raw coal slurry at a rate of one-three pounds of additive per ton of coal, depending on the particle size distribution of the raw coal and the molecular area of the additive.
Dispersion of the coal particles in the liquid carrier in the second of the milling steps can also be promoted in many cases by maintaining the pH of the slurry in the range of 6-10 during that step. This can be accomplished by adding a basic material such as sodium hydroxide to the slurry in an amount that increases the pH of the slurry to the desired level.
Reductions in ash content to the levels envisaged herein require an agglomerating agent of particular character; viz., one that has an exceptionally high interfacial tension with water (at least 50 dynes/cm and the higher the better) and a reasonably low viscosity. Agglomeration of the product coal particles in the disclosed fuel preparation process involves attachment of the agglomerant to the particles of coal liberated in the milling steps and the formation of liquid agglomerant bridges between the particles making up each agglomerant. If the interfacial tension between the agglomerant and the aqueous phase of the coal slurry is not at least 50 dynes per cm, microspheres (or bubbles) of water and mineral matter can fill the voids between and around the coal particles making up the agglomerates. This undesirably increases both the moisture and ash content of the product coal. By using an appropriate amount of an agglomerant having an interfacial tension with water of the magnitude identified above, however, the filling of the voids with agglomerant and the ejection of water and mineral matter from those voids into the main body of the slurry can be insured.
Suitable agglomerants for my purposes include such diverse compounds as pentane, 2-methylbutane, 1,1,2-trichloro-1,2,2-trifluoroethane, and trichlorofluoromethane. Essentially pure compounds are required as even small amounts of impurities markedly lower the interfacial tension of the agglomerant with respect to water.
The agglomerant forms stable, monolayer films on the coal particles, rendering the particles more hydrophobic relative to the water phase. The amount of agglomerant needed to achieve a monolayer film can be readily calculated from the area of the coal particles and the area of the specific agglomerant molecules. Similarly, the amount of agglomerant required to achieve separation of up to essentially all of the product coal with low ash contents (typically below one percent) from the mineral water slurry can be calculated using as a first approximation the packing of ideal spheres and the change of the agglomerant film thereon to determine that point where the agglomerant attached to the coal particles just, but completely, fills all of the voids between all of the coal particles, yielding a minimum area for the high energy interfacial contact between the agglomerant and the water in the raw coal slurry.
In the case of 1,1,2-trichloro-1,2,2-trifluoroethane, ca. 0.19 wt % of the agglomerant based on dry coal weight will suffice to form the stable monolayers on the coal particles whereas 45 wt % or more of the agglomerant will have to be dispersed in the diluted raw coal slurry to completely fill the voids between the coal particles making up the product coal agglomerates. Separation over a sieve bend can be readily achieved, and most often the optimum reduction of ash in the product coal (depending on the coal and the size distribution) can be observed, when very near 55 wt % agglomerant has been dispersed on the coal particles. Agglomerant in excess of 65 wt % based on dry coal results in partial or complete separation of one slurry containing liquid agglomerant and product coal from a second slurry of water with mineral matter.
Petroleum fractions such as Varsol, kerosene, and gasoline are occasionally reported as having interfacial tensions with water in the range of 50 dynes/cm. However, these cuts usually contain acids, ketones, and unsaturated and other compounds that effectively lower this value. Consequently, these and comparable cuts such as light hydrocarbon oils heretofore proposed as agglomerants can not be used to reach the goals of the present invention--the generation of a product from raw coal which has minimal ash and pyritic sulfur at recovery rates approaching 100 percent.
One important advantage of the novel agglomerants employed in the practice of the present invention, aside from their high interfacial tensions with water, is that they have a boiling point below that of water. This is particularly important when agglomeration and separation of the product coal is followed by redispersion of the coal particles in clean water, reagglomeration, and separation. Redispersion requires that the concentration of agglomerant with respect to the solids in the agglomerates be reduced in the presence of an aqueous carrier. That cannot be accomplished if the boiling point of the agglomerant is above 100.degree. C. as the carrier will boil off before the agglomerant is evaporated if heat is added to the mixture of agglomerates and water to strip off the agglomerant.
The exemplary agglomerants identified above all have boiling points in the range of 30.degree.-50.degree. C. Agglomerants in that boiling point range are especially desirable as they remain liquid under most ambient conditions but can be dissociated from the product coal and the water-mineral matter phase of the slurry with only modest expenditures of energy. This is important as the cost of the large volume of agglomerant used in a commercial scale operation requires that essentially all of the agglomerant be recovered and recycled.
Another advantage of the preferred class of agglomerants is that they have viscosities of less than one centipoise. This is important because, as a consequence of their low viscosity, those agglomerants can be easily and therefore economically dispersed in the slurry in a manner that will produce the requisite encapsulation of the coal particles by the agglomerant. Specifically, the transport of the liquid agglomerant from the water-solids-agglomerant mixture to the product coal occurs by the impact of dispersed agglomerant on the coal particles and the subsequent wetting of the coal particles by the agglomerant. This process, which tends to homogenize the agglomerant distribution over all of the particles, requires that the viscosity of the agglomerant be below 1000 centipoises; and the process becomes increasingly more efficient as the viscosity decreases below that maximum value.
Another advantage of the agglomerants I employ in addition to their efficacy is that they do not react with coal which is important for the reasons discussed in U.S. Pat. No. 4,173,530 issued Nov. 6, 1979, to Smith et al.
Several advantages of the novel fuel preparation processes described herein have been described above. Another is that they can be employed to produce fuels from raw coals ranging from sub-bituminous through bituminous to anthracite as well as from lignite which has above and will hereinafter also be included in the term "coal" for the sake of convenience.
High ranked, unoxidized coals have a natural hydrophobicity and can be treated by the agglomeration type separation process as described above.
Partially oxidized coals and coals of lower rank, however, lack this natural hydrophobicity to at least some extent because of their oxygen content. Hydrophobicity to the desired extent can be induced in such coals by using a surfactant to modify the naturally hydrophilic surfaces of the coal and, in effect, transform it into a hydrophobic coal that responds to the process in the same manner as one that is naturally hydrophobic.
The surface active agent, which may be oleic acid or one of its soluble salts, is preferably mixed with the slurry prior to the separation and agglomeration of the product coal particles in an amount sufficient to produce a monolayer of surfactant on the coal. The carboxylic acid (or comparable) group of the surface active agent attaches to the polar surface of the coal, allowing the molecule to establish an apparent coal surface which is repulsive to water because of induced hydrophobicity but possesses a strong attraction to the agglomerant. This allows the lower rank or partially oxidized coal particles to be dissociated from the mineral matter and aqueous phases of the slurry and then agglomerated in the same manner as unoxidized coals of higher rank.
Excess surfactant must be avoided, however, as the excess will significantly reduce the interfacial energy between the agglomerant and the water in the slurry, causing an increase in the ash content of the product coal agglomerates. To avoid this same undesirable result, care must be exercised to avoid the use of surfactants that would render the surfaces of the mineral matter particles in the slurry hydrophobic.
Strong Lewis bases can also be employed to induce hydrophobicity in partially oxidized and lower ranked coals. Lewis bases can be combined into a single molecule with a hydrophobic, organic chain or ring. The Lewis base moiety of the molecule attaches the latter to the coal particles, and the organic fractions of the compounds form a monolayer of additive that renders the entire surface of each coal particle hydrophobic. Those surfaces accept the agglomerating agent in a manner identical to that characteristic of an unoxidized, high ranked coal.
Lewis base-containing molecules that can be employed for the purposes just described are those of the formulas R-OH, R.sub.2 -NH.sub.3, R-NH.sub.2, and R.sub.3 N where R is an organic chain or ring with more than four hydrocarbons.
An alternative to inducing hydrophobicity is to increase the agglomeration time for partially oxidized and/or lower rank coals. Unoxidized, high rank coals can be completely agglomerated in periods of &lt;5-15 seconds. By increasing the time to minutes, many oxidized coals can also be successfully agglomerated although others cannot because agglomeration time increases with the state of oxidation, reaching infinity for a fully oxidized coal.
It was pointed out above that U.S. Pat. No. 4,186,887 discloses a process having some similarities to the novel fuel preparation processes disclosed herein. There are also significant differences.
For example, the fuel preparation processes described herein differ from the coal recovery process described in U.S. Pat. No. 4,186,887 in that there is no milling during the coal recovery phase of the process in which the coal particles are dissociated from the mineral matter and aqueous phases of the slurry in which they are found and then coalesced into product coal agglomerates. This is significant because it has been found that wear--for example, of the balls in a ball mill--by prolonged milling continued into the recovery phase can result in enough worn away material being agglomerated with the coal to significantly increase the ash content of the latter.
The novel fuel preparation processes disclosed herein also differ significantly from the coal beneficiation process described in U.S. Pat. No. 4,186,887 in that the addition of the agglomerant to the coal slurry and the subsequent dissociation of the coal particles from the mineral matter and aqueous phases of the slurry and coalesence of those particles into agglomerates are preferably carried out separately.
As discussed above, the essentially complete separation of the coal particles from the associated mineral matter achieved by the fuel preparation processes described herein requires that a monolayer of the agglomerant be adsorbed on the surface of each coal particle. This can most efficiently be achieved in a different unit than the subsequent separation of the product coal from the slurry because the dispersion of the agglomerant is a kinetic process requiring a finite period of time. By carrying out this step separately, one can insure that the wanted dispersion of the agglomerant is completed before the separation of the product coal from the agglomerant is attempted.